DESCRIPTION The aims of the proposed study are to observe the natural history of Helicobacter pylori infection in infants of low-income families living on the US-Mexico border and to identify factors that influence acquisition and persistence of infection. Although it is known that H. pylori infection is associated with low socioeconomic status, little evidence to date addresses the influence of environmental factors on the risk of acquiring the infection as opposed to the influence of these factors on the probability of having a persistent infection. Specifically, this study aims to: 1) estimate the incidence of H. pylori infection during the first three years of life in two cohorts of infants, one on each side of the Rio Grande river in San Elizario, El Paso County, Texas and Juarez, Mexico; 2) determine the frequency with which H. pylori infection spontaneously clears in infants in these cohorts; and 3) estimate the effect of socioeconomic indicators, hygiene, and diet on H. pylori incidence, recurrence and persistence. In San Elizario, Texas, infants will be recruited from those attending the WIC clinic for well child care. In Juarez, infants will be recruited from day care centers operated by the Instituto Mexicano del Seguro Social. A baseline questionnaire will be administered to the infants' mothers to solicit information on relevant socioeconomic and hygienic factors. Infants will be tested for H. pylori infection initially at 6 months of age and at 6 month intervals thereafter through the third birthday. The 13C-urea breath test will be used to detect active infection at the time of testing. In addition, serology will be used as evidence of transient infection that may have occurred during the preceding 6- month interval. Hygiene-related exposures and diet will be assessed at each 6-month follow-up visit. Data collection and analyses will be designed to address the following specific causal hypotheses: 1) indicators of low socioeconomic status of parents will be linked to an increased risk of H. pylori infection during infancy; 2) factors that facilitate direct person-to- person transmission of infectious agents increase the risk of H. pylori infection during infancy; 3) exposure to unpurified water for drinking, bathing or swimming increases the risk of H. pylori infection during infancy; 4) breast feeding decreases the risk of H. pylori infection during infancy; 5) inadequate nutrition increases the probability of persistent H. pylori infection during infancy; 6) frequent exposure to antibiotic therapy decreases the probability of persistent H. pylori infection during infancy.